1. Field of the Invention
The present invention relates to a signal amplification device and, more particularly, relates to an amplification device which can be driven at a low voltage.
2. Description of the Related Art
A conventional amplifier may have a circuit constitution such as shown in FIG. 1. In such an amplifier, a charge signal obtained by a CCD (Charge Coupled Device) image sensor is converted into a voltage signal through a capacitance in a floating diffusion layer. The voltage signal is amplified by a source follower amplifier on a sensor chip and is then output from the sensor. A reset level of the floating diffusion layer periodically appears in output signals from the sensor. Accordingly, a correlated double sampling (CDS) circuit extracts the portion corresponding to a pixel signal among the output signals. The output signal extracted by the CDS circuit is further amplified by an amplifier using a differential amplifier circuit. In other words, such an amplifier includes a differential amplifier circuit 2 using a current mirror as a load. A reference voltage VSTD is supplied to the gate of an MOS transistor MN02 as one input terminal. A voltage signal VSIG as a CCD output signal is supplied to the gate of an MOS transistor MN01 as the other input terminal. The differential amplifier circuit 2 generates an output signal according to a difference between the two supplied voltages. Because the differential amplifier circuit 2 uses a current mirror circuit as a load, the circuit 2 functions as a transconductance type amplifier. That is, a voltage signal is supplied as an input signal but a current signal is obtained as an output signal. The generated current signal is converted into a voltage signal VOUT by a resistor RL coupled to an output terminal 4 of the differential amplifier circuit 2. The operating point of the signal VOUT is determined in accordance with a constant current which is supplied from a transistor MN04 (gate bias voltage VB2) as a constant-current source to the resistor RL.
In the conventional amplifier shown in FIG. 1, the potential of the output terminal 4 of the differential amplifier circuit 2, namely, a node between the drain of the transistor MN02 and the drain of a transistor MP02 is equal to that of the voltage signal VOUT. The potential is varied in accordance with the signal VOUT. In the differential amplifier circuit 2, however, a plurality of transistors are arranged in series between a power supply voltage VDD and ground potential GND. Specifically, three MOS transistors of the transistor MP02 of the current mirror circuit, the transistor MN02 of the differential amplifier circuit, and a transistor MN03 (gate bias voltage VB1) as a constant-current source are connected in series. To operate, each transistor requires a predetermined potential difference between the source and the drain. The remainder obtained by subtracting the potential difference from the VDD-GND voltage is a voltage fluctuation allowance xcex94VOUT which is allowable at the output terminal 4. In the example conventional amplifier, the voltage VDD is set to a relatively large value, for example, +5V. In accordance therewith, the voltage fluctuation allowance xcex94VOUT is held enough.
In order to realize a reduction in the size of various portable devices, it is preferable that batteries of smaller size are used. Accordingly, power consumption by devices has been reduced so that the devices can operate for a sufficient time even when equipped with a small battery having a small capacity. In a digital still camera or video camera, such reduced power consumption and size reduction are desired. Meanwhile, as communication capacity has increased, it has become possible for portable telephones and portable terminals to receive and transmit images. As a result, there is a growing demand for mounting a photographing function on such devices. Because the portable telephones is small, a reduction in power consumption is further required in association with the small size. As one way of reducing power consumption, the CCD output signal processing circuit can be driven at a low voltage, namely, the power supply voltage VDD is reduced. When this is done, however, it is difficult to reduce the potential difference between the source and the drain of a transistor. The amount of reduction in the voltage VDD is fundamentally passed to the allowance xcex94VOUT. Conversely, such a conventional amplifier has certain problems including that it is difficult to improve low-voltage driving while maintaining the amplitude of an output voltage signal and that power consumption cannot always be reduced as desired.
The present invention is made to solve the above problems. It is an object of the present invention to provide a signal amplification device in which low-voltage drive is realized while maintaining the amplitude of an output voltage signal, such that power consumption is reduced.
According to the present invention, there is provided a signal amplification device including a differential amplifier circuit which generates a first output according to an input signal and a current mirror circuit which receives the first output of the differential amplifier circuit to an input-side current path to generate a mirror current through an output-side current path in accordance with a current flowing through the input-side current path, wherein the mirror current is converted into an output voltage by a resistive load in the output-side current path of the current mirror circuit.
According to the present invention, the first output generated from the differential amplifier circuit is not converted into a voltage signal in principle at an output terminal of the differential amplifier circuit but is normally generated as a current signal, so that any fluctuation in potential at the output terminal is suppressed. Accordingly, the first output generated from the differential amplifier circuit is supplied to the input-side current path of the current mirror circuit. The current mirror circuit generates a mirror current through the output-side current path in accordance with the input-side current flowing through the input-side current path. In other words, the mirror current also fluctuates in accordance with the current fluctuation of the first output from the differential amplifier circuit in the input-side current path. As noted above, the current signal is transmitted to the output-side current path of the current mirror circuit. To suppress the potential fluctuation at the output terminal of the differential amplifier circuit, fundamentally, a resistor is not disposed in the path through which the current signal flows, in the input-side current path. Rather, a resistor is provided as a load in the output-side current path. A fluctuation in mirror current signal is converted into a voltage fluctuation across the resistor. The voltage fluctuation is extracted as an output voltage signal of the present voltage signal amplification device. A driving voltage for the differential amplifier circuit and the input-side current path can be reduced as much as the suppressed amount of voltage fluctuation at the output terminal of the differential amplifier circuit. Only a single transistor in the output-side current path is required for controlling a mirror current signal in accordance with a signal from the input-side current path. On the other hand, a plurality of transistors may be serially connected between power supply potential and ground potential in the differential amplifier circuit or the input-side current path. That is, the amplitude of the output voltage signal generated in the output-side current path can be large because there are fewer transistors in the output-side current path than in the differential amplifier circuit or the input-side current path. Accordingly, the driving voltage in the output-side current path can be reduced while the amplitude of the output voltage signal is maintained.
According to the present invention, the signal amplification device may be constituted in such a manner that the current mirror circuit comprises a potential setting circuit for holding the potential on the input side at a predetermined value.
The current mirror circuit comprises a pair of transistors whose gates (or bases) are connected to each other in the input-side current path and the output-side current path. A source-drain (emitter-collector) potential difference of the transistor disposed in the input-side current path can be fluctuated in accordance with a current on the input side. Generally, a fluctuation in potential difference is smaller than voltage drop in a simple resistor element. For example, potential at a node between the differential amplifier circuit and the input-side current path may fluctuate as explained above. According the present invention, however, the above-mentioned constitution includes means for holding the potential at the node to a predetermined value. Consequently, the voltage fluctuation at the output terminal of the differential amplifier circuit is fundamentally eliminated, so that the driving voltage for the differential amplifier circuit and the input-side current path can be reduced as much as the eliminated voltage fluctuation.
The signal amplification device according to the present invention may be constituted in such a manner that the current mirror circuit includes a constant-current source which supplies a constant current to the input-side current path such that a current signal from the differential amplifier circuit is superimposed on the supplied current from the constant-current source to form an input-side current.
With such a constitution, the input-side current flowing through an input-side transistor constituting the current mirror circuit is a current obtained by combining the supplied current from the constant-current source with the current signal from the differential amplifier circuit. That is, the input-side current causes a fluctuation in range of the current signal in the vicinity of the supplied current. When the supplied current from the constant-current source is changed, the operating point of the output voltage signal generated in the output-side current path can be adjusted. The source-drain (or emitter-collector) potential difference of the input-side transistor constituting the current mirror circuit can be fluctuated in accordance with the current signal from the differential amplifier circuit as mentioned above. The fluctuation allowance can be more suppressed as the input-side current, which is the supplied current, is larger. Accordingly, when the current is supplied from the constant-current source to the input-side current path, potential fluctuation at the node between the differential amplifier circuit and the input-side current path is suppressed, and the driving voltage for the differential amplifier circuit and the input-side current path can therefore be reduced.
According to the present invention, preferably, in the voltage signal amplification device, the current mirror circuit includes, as the only transistor in the output-side current path, a mirror-current generating transistor connected to the input-side current path to generate the mirror current. Consequently, while the amplitude of the output voltage signal is maintained, the output-side current path can be driven at the same voltage as the driving voltage needed for the differential amplifier circuit and the input-side current path.